What is the significance of the scene where Karen and Monnitoff briefly talked about Donnie Darko?

I've never thought the tone in that scene was disapproving so much as it is knowing. Both teachers have had interesting interactions with Donnie, and they both know something is up with that kid. I've wondered if they each suspected that Donnie was involved in the vandalism at the school earlier in the film, but that's reading a lot into a scene with only four words of dialogue.

I believe this scene goes hand in hand with the theory of Manipulated living and Monnitoff and Karen might know some things about what is going on.

The fact that these two people make significant contributions in how the plot unfolds can be read together with this.

Actions by Karen

• Seating Gretchen next to Donnie, so they will end up together
• Writing Cellar Door on the blackboard

Actions by Monnitoff

• He gives Donnie the book The philosophy of time travel (which is not a recommended book to learn about time travel) (and asks him to keep it a secret).
• When Donnie is talking about the "Channels" of time travel with his Science teacher, and begins to really figure it out, his teacher says to him, "I'm not able to continue this conversation.

Some of these actions that seems trivial is turning out to be some of the most important things in the storyline. So my inference is either these people are guiding Donnie to what he should do or because of the number of times the time loop has happened, some of the manipulated living remember the correct decisions and do them subconsciously. This scene also goes along with that hypothesis.